Egypt's powerful army called for President Mohamed Morsi and the secular opposition to meet to resolve a deepening crisis over a constitutional referendum that sparked rival mass protests on Tuesday.

General Abdel Fattah al-Sissi, the country's armed forces chief and defense minister, made the appeal "for the sake of Egypt" for all political groups and movements to meet on Wednesday at a Cairo military sports complex, according to a statement posted on the military's official Facebook page.

It came as tens of thousands of protesters filled Cairo streets to demonstrate for and against the referendum called for Saturday on a draft constitution backed by Morsi and his Islamist allies.

There were fears the rival rallies could mix, sparking clashes like those seen outside the presidential palace last week, when seven people were killed and hundreds injured in a melee between mobs wielding metal bars, petrol bombs and handguns.

Troops have orders from the president to use police powers to protect "vital state institutions".

Outside the palace, thousands of opposition protesters tore down a metal and concrete barricade to denounce the Saturday referendum, forcing hundreds of soldiers back but without violence.

"We are here to say: 'Down with the illegitimate constitution'... If the referendum happens we will have to vote. But hopefully it won't," said one protester, Ahmed Badawy, 29.

At a much bigger Islamist counter-demonstration a few kilometers (miles) away gathering tens of thousands of referendum supporters, determination was equally evident.

"It's the last battle for Islam against the secularists who want to ruin Egypt," said Ahmed Alaa, who was bussed in from the north of the country.

Around him the crowd held up banners saying "Yes to the constitution," and waved Saudi and black Islamist flags as well as the Egyptian one.

The military, which has vowed to maintain stability while trying to remain neutral, has been caught in the middle of the dispute.

The main opposition bloc, the National Salvation Front, has so far ignored a previous call the army made at the weekend to start dialogue, saying talks are not possible unless the referendum is called off.

A key group of judges said on Tuesday that they would refuse to oversee the plebiscite. It was not known whether other judges would follow their lead.

Political analyst Emad Gad said that if no solution was found, there was a chance the crisis might prompt the army to step in and maybe even seize back the political control they gave up on Morsi's June election.

"In the event there are violent clashes or especially if blood is spilt in the street, the army will certainly intervene," he said.

The opposition, made up of secular, left-wing and liberal groups, sees the draft constitution rushed through by an Islamist-dominated panel last month as weakening human rights, the rights of women and religious minorities.

The U.N. human rights chief and international watchdogs have criticized the draft and the way it was drawn up.

Morsi's supporters, however, argue that it is up to Egypt's voters to decide in the referendum.

Michael Wahid Hanna, a political analyst at U.S. think tank The Century Foundation told Agence France Presse that, as things stood, there was a good chance of the referendum passing.

If that happened, Hanna warned, "I fear they are going to have an institutionalized crisis" that would polarize Egypt in the long term, raising "the specter of violence".

The prolonged crisis, the worst since a popular uprising overthrew autocratic president Hosni Mubarak early last year, is intensifying uncertainty over Egypt's economy.

The International Monetary Fund on Tuesday put a proposed $4.8 billion loan on hold.

Egyptian Prime Minister Hisham Qandil said: "We have officially requested the delay of a month in the negotiations with the IMF because of the political situation in the country."

The IMF's executive board had been expected this month to review a provisional agreement on the loan, which is needed to bridge financing shortfalls through fiscal 2013-2014 as the country rebuilds its battered post-revolution economy.